1. Field of the Invention
The present invention relates to piezoelectric single crystal wafer, particularly to piezoelectric single crystal wafer used for fabrication of surface acoustic wave or leaky surface acoustic wave device.
2. Description of the Related Art
Surface acoustic wave device or leaky surface acoustic wave device is a circuit element that convert electric signal respectively to surface acoustic wave or leaky surface acoustic wave to conduct signal processing. They have been conventionally used for a filter, a resonator, a delay line or the like.
Such a surface acoustic wave device or leaky surface acoustic wave device is generally produced by forming electrodes for transmit-receive of surface acoustic wave or leaky surface acoustic wave on a piezoelectric single crystal wafer and cutting it in a form of a chip.
The above-mentioned piezoelectric single crystal wafer itself is generally produced as follows. First, piezoelectric single crystal having a piezoelectricity is grown by an adequate method for growing a single crystal. For example, a crystal ingot made of lithium tantalate is grown by Czochralski method (CZ method). Then, the single crystal ingot is subjected to a cylindrical grinding so that a section vertical to an axis thereof may have a round shape. The resultant cylindrical single crystal ingot is sliced to provide a wafer having a constant crystal surface orientation. Then one side or both sides of the wafer produced thereby was subjected to lapping, mirror polishing in sequence. Thereby, the piezoelectric single crystal wafer can be produced.
On the surface subjected to a mirror polishing of the piezoelectric single crystal wafer thus produced, an electrode mainly made of Al is formed in a regular direction (hereunder the surface on which the electrode is formed is occasionally called xe2x80x9can electrode forming surfacexe2x80x9d or xe2x80x9ca front surfacexe2x80x9d) and cut in a shape of a chip. Thereby, the surface acoustic wave or leaky surface acoustic wave device can be fabricated.
Performance of the surface acoustic wave or leaky surface acoustic wave device produced as above depends on various factors such as material to be used, crystal orientation, design of electrodes, conditions for fabrication or the like. The factor for performance of the surface acoustic wave or leaky surface acoustic wave device that is particularly to be noted is a surface work damage layer existing at a depth of tens xcexcm or less from the surface of the wafer.
The relation between surface acoustic wave and a surface work damage layer, and a method of polishing a single crystal wafer for a surface acoustic wave considering a surface work damage layer have been reported as follows.
Kimura et al. (Kimura et al., Shingaku Gihou, US 75-56, 17-23, 1975) reported that a relation between the surface work damage layer and Q factor of surface acoustic wave was investigated using quartz, and it was found that when a significant surface work damage layer such as micro crack was present to a depth of a half of wavelength, it was confirmed as difference of the above-mentioned Q factor of surface acoustic wave.
Kimura et al. (T. Kimura et al., J. Appl. Phys., 50 (7), 4767-4772, (1979)) reported that a relation between a surface work damage layer and propagation loss was investigated and analyzed using quartz, and reported a relation between a depth and an amount of micro cracks and propagation loss.
As for a method of polishing a lithium tantalate single crystal wafer for surface acoustic wave, there have been reported as a similar method used for processing of a silicon single crystal wafer, a polishing method wherein SiO2 colloidal polishing agent is used after a lapping process (Nihon Gakujutushinkoukai Danseihyoumenhasoshigijutu, 150th committee, acoustic wave element handbook, 296-298 (1991)). It has been reported that it is important in a polishing step to obtain a state of a mirror surface and completely remove a mechanical damage layer generated in a lapping step, and that polishing with a stock removal of 10 xcexcm or less is sufficient, since abrasive grains having a diameter of 15 xcexcm or less is generally used for lapping, and thus a mechanical damage layer is generally formed at a depth of 10 xcexcm or less.
As described above, there have been previously reported a relation between a surface acoustic wave and a surface work damage layer, or a method of polishing a single crystal wafer for surface acoustic wave considering a surface work damage layer. However, there have not been reported a relation between leaky surface acoustic wave and a surface work damage layer, or a method of polishing a single crystal wafer for leaky surface acoustic wave.
There is a significantly increased demand for surface acoustic wave or leaky surface acoustic wave device as a filter for mobile communication that has been diffused explosively in these years. There is also a demand for improvement in quality thereof, especially a demand for improvement in uniformity of surface acoustic wave velocity or leaky surface acoustic wave velocity on the surface of the piezoelectric single crystal wafer used for a device, and the uniformity among wafers.
The filter for mobile communication is required to be improved not only in quality, but also in cost performance with increase of a demand. The yield in production of the filter have much influence on production cost.
The relation between surface acoustic wave velocity or uniformity thereof and composition, cutting method or propagation direction has been reported (Nihon Gakujutushinkoukai Danseihyoumenhasoshigijutu, 150th committee, surface acoustic wave handbook, 289-302, (1991), and Aikawa et al., Denshijouhoutsushingakkai shuukitaikai, 19 (1994)).
However, there comes to be a problem of a deviation of surface acoustic wave velocity or leaky surface acoustic wave velocity on the surface of the piezoelectric single crystal wafer, and among the wafers, which is considered unable to be explained only with composition, a cutting method, or propagation direction.
There are various factors for failure in production of the filter. Many of them are caused by fine contaminations (particles) adhered on the surface of the wafer or due to breakage of the wafer. The recent filter for mobile communication is often used in a range of frequency of 0.1 to 3 GHz, especially 1 to 2.5 GHz. Accordingly, an electrode having a line width of 1 xcexcm or less has been requested. Therefore, the fine contaminations having a size of several xcexcm that did not matter previously are getting a big problem. For example, if an electrode is formed on the wafer on which contaminations having a size of several xcexcm are adhered, the electrode cannot be formed at a part on which the contaminations are adhered, and thus desired propagation properties cannot be achieved. As a result, yield is lowered, and production cost is increased.
The present invention has been accomplished to solve the above-mentioned problems, and the first object of the present invention is to provide a piezoelectric single crystal wafer for surface acoustic wave or leaky surface acoustic wave device wherein a deviation of surface acoustic wave velocity or leaky surface acoustic wave velocity is small, namely the velocity uniformity is excellent. The second object of the present invention is to provide a piezoelectric single crystal wafer wherein fine contaminations adhered on the surface on which an electrode is formed and breakage of the wafer are significantly reduced, and to provide a piezoelectric single crystal wafer that can provide a filter or the like having an excellent property in high production yield.
The inventors noted and studied from the following two reasons a relation between a deviation of surface acoustic wave velocity or leaky surface acoustic wave velocity and a surface work damage layer.
One of the reasons is that although a relation between Q factor or propagation loss of surface acoustic wave and a surface work damage layer has been reported as described above, a relation between a deviation of surface acoustic wave velocity or leaky surface acoustic wave velocity and a surface work damage layer has not been reported.
The other of the reasons is that it is necessary to consider the surface work damage layer other than micro crack in order to meet the recent requirement for quality.
Although the recent requirement for quality is quite different from the previous requirement, a method for processing and an approach therefor have not been changed. Accordingly, the inventors of the present invention have considered that a state of a part near the surface at which energy of surface acoustic wave or leaky surface acoustic wave is concentrated, namely the surface work damage layer is quite important. The word xe2x80x9csurface work damage layerxe2x80x9d herein means not only micro crack but also mechanical damage or the like. The inventors have found that measurement of etching pit density is effective as a method of quantitative evaluation of the surface work damage layer. As a result, they have found a correlation between an etching pit density and surface acoustic wave velocity or leaky surface acoustic wave velocity.
In order to achieve the above-mentioned first object, according to one aspect of the present invention, there is provided a piezoelectric single crystal wafer wherein an etching pit density on the front surface of the wafer on which an electrode for transmit-receive of surface acoustic wave or leaky surface acoustic wave is formed is 7.8xc3x97104/mm2 or less.
As described above, if an etching pit density on the front surface of the wafer on which an electrode for transmit-receive of surface acoustic wave or leaky surface acoustic wave is formed is 7.8xc3x97104/mm2 or less, a deviation of surface acoustic wave velocity or leaky surface acoustic wave velocity in production of the device is small, the piezoelectric single crystal wafer for surface acoustic wave or leaky surface acoustic wave device having an excellent propagation property can be provided.
The inventors have also studied and analyzed further fine contaminations adhered on the surface of the wafer, and found that a lot of fine particles consisting of the same material as a single crystal constituting the wafer are adhered on the surface. They have also found that more of the fine particles consisting of the same material as the single crystal are observed at a part nearer the periphery of the wafer. They have studied further occurrence of the fine particles, and found that the piezoelectric single crystal wafer is very easy to break compared with a wafer such as a silicon wafer. Therefore, if there are fine unevenness at peripheral surface of the wafer, the prominence may be broken with only small impact to generate fine particles that will be adhered on the part near the periphery of the wafer. They have also found that breakage of the wafer occurred at the part having fine unevenness on the peripheral surface of the wafer.
According to the above-mentioned findings, the inventors have found that adhesion of the fine particles on the part near the periphery of the wafer can be suppressed and breakage of the wafer can be prevented by providing the piezoelectric single crystal wafer described below. They also have found that a device such as a filter or the like having an excellent propagation property can be produced in high yield by fabricating surface acoustic wave or leaky surface acoustic wave device using such a wafer, and completed the present invention.
In order to achieve the second object of the present invention, according to the second aspect of the present invention, there can be provided a piezoelectric single crystal wafer wherein surface roughness Ra on the peripheral surface other than the front surface of the wafer where an electrode for transmit-receive of surface acoustic wave or leaky surface acoustic wave is formed and the reverse surface thereof is 2.3 xcexcm or less.
As described above, if the surface roughness Ra on the peripheral surface of the wafer is 2.3 xcexcm or less, the peripheral surface of the wafer is very smooth, so that the wafer can be prevented from being broken, and there is almost no prominence that may cause generation of fine particles. Accordingly, occurring of crack can be prevented almost completely, during transportation and vacuum in a film-forming step for forming an electrode on the surface of the wafer or in a photolithography step, so that breakage of the wafer can be prevented. Furthermore, there is almost no fine particles generated from the wafer itself, especially on the peripheral surface thereof, that is scraped due to rub with a container, handling or the like. As a result, fine particles adhered on the surface of the wafer are significantly reduced.
On the surface of the piezoelectric single crystal wafer having such a small surface roughness of the peripheral surface, almost no fine contaminations (particles) are adhered, so that there is no part where an electrode is not formed due to presence of the fine contaminations when the electrode is formed on the surface. As a result, a device such as a filter having excellent propagation property can be produced in high yield.
According to the present invention, it is preferable that the above-mentioned piezoelectric single crystal wafer is made of lithium tantalate, lithium niobate, lithium tetraborate, langasite, or quartz. Especially, the piezoelectric single crystal wafer made of lithium tantalate single crystal has well balanced propagation property of surface acoustic wave or leaky surface acoustic wave.
More preferably, the above-mentioned piezoelectric single crystal wafer is a lithium tantalate single crystal wafer wherein the surface of the wafer on which an electrode is formed is parallel to X axis, and a perpendicular line to the surface of the wafer and Y axis make an angle in the range of 33 to 46xc2x0 (hereinafter referred to as 33-46xc2x0 Y cut) wherein X axis is a-axis of the single crystal, and Z axis is c-axis thereof, and Y axis is a vertical axis to the X axis and Z axis.
Thus, if the piezoelectric single crystal wafer is the above-mentioned lithium tantalate single crystal, the piezoelectric single crystal wafer shows a further excellent leaky surface acoustic wave propagation property when the electrode is formed on the surface of the wafer on which fine particles are not adhered.
As described above, the piezoelectric single crystal wafer for surface acoustic wave or leaky surface acoustic wave device according to the first aspect of the present invention is characterized in that an etching pit density on the surface of the wafer where an electrode for transmit-receive of surface acoustic wave or leaky surface acoustic wave is formed is 7.8xc3x97104/mm2 or less, and the surface acoustic wave or leaky surface acoustic wave device fabricated using the wafer has a very small deviation of the surface acoustic wave velocity or leaky surface acoustic wave velocity and an excellent propagation property.
The piezoelectric single crystal wafer of the second aspect of the present invention is characterized in that the surface roughness Ra on the peripheral surface is 2.3 xcexcm or less, namely very smooth. Accordingly, almost no breakage of the wafer occurs during transportation and vacuum in a film forming process for forming electrodes on the surface of the wafer or in a photolithography process or the like. There are almost no fine particles generated from the peripheral surface of the wafer ground due to rub with a container, handling or the like. As a result, fine particles adhered on the surface of the wafer is significantly reduced.
As described above, there is almost no fine contamination adhered on the surface of the piezoelectric single crystal wafer having low surface roughness on the peripheral surface. Accordingly, if an electrode is formed on the surface, there is no part where the electrode is not formed due to fine contaminations, so that the device such as a filter having excellent properties can be produced in high yield.